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Advice on reshaping & sharpening an Usuba knife

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Unfortunately I noticed the edge of the knife isn't straight: when the edge is placed on a flat surface some parts of the blade touch the table whereas other parts don't. This can clearly be seen in the following photo:

I guess I can just try to 'reshape' the edge (starting with a rough whetstone), but I expect this is going to be a pretty hard job. Is there a good 'standard' technique for this or does somebody has some good advice for me?

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Next to this, I have another related question: when sharpening the usuba I place the entire 'blade path' (kiriba) on the whetstone when sharpening the front side. As I understood (correct me if I'm wrong), this (see cartoon below) is the way to sharpen a single bevel knife:

A problem arising from using this techniques seems to be that I introduce scratches on the 'blade path'/kiriba-part as pointed out by the red arrow in the photo below. I find these scratches unappealing, and would like to learn to sharpen a knife without scratching it in this way.

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So my second questions is: can this usuba be sharpened without introducing these scratches or should these scratches be removed by polishing after sharpening? Or is this (cheap) usuba possibly just of low-quality and will these stains/scratches not appear when I sharpen an expensive usuba in the same way?

The coarse scratches are removed by using finer abrasives which leave finer scratches. Have you checked the knife is straight? In my experience Cheaper usuba a are often slightly out of straight, which introduces waviness in the edge when sharpening.
Watch Jon from JKIs videos on YouTube for a primer on single bevel sharpening

Unfortunately I noticed the edge of the knife isn't straight: when the edge is placed on a flat surface some parts of the blade touch the table whereas other parts don't. This can clearly be seen in the following photo:

I think the section toward the "tip" of the usuba usually isn't in the same line as the rest of the edge, so this seems normal to me. The gap towards the heel/choil of the edge isn't right though.

You have to sharpen it that way in order to maintain the knife's proper geometry as you remove steel and the edge rises up towards the spine. In addition when you sharpen the blade road you apply pressure directly over the shinogi line, then you sharpen while applying pressure directly over the lamination line, then you blend the two together. This creates a compound bevel called a hamaguriba which translates to clamshell due to it's shape. Finally you uraoshi sharpen which means to push the back side because you are only to apply slight pressure on the push so as to keep the ura even around the blade since pulling backwards naturally adds a downward force. As far as the scratches, you have to get some decent stones which leave that misty or kasumi look when finished. I use the gesshin 400- king 800- gesshin aoto on the blade road to achieve this then I finish the hagane and ura up to the rika 5k.

The coarse scratches are removed by using finer abrasives which leave finer scratches. Have you checked the knife is straight? In my experience Cheaper usuba a are often slightly out of straight, which introduces waviness in the edge when sharpening.
Watch Jon from JKIs videos on YouTube for a primer on single bevel sharpening

Thanks for the advice! The highest grid stone I have is a 3k, but I'm gonna see what I can do. What do you mean exactly with if the knife is straight? Does the first photo in my post not answer that question?

You have to sharpen it that way in order to maintain the knife's proper geometry as you remove steel and the edge rises up towards the spine. In addition when you sharpen the blade road you apply pressure directly over the shinogi line, then you sharpen while applying pressure directly over the lamination line, then you blend the two together. This creates a compound bevel called a hamaguriba which translates to clamshell due to it's shape. Finally you uraoshi sharpen which means to push the back side because you are only to apply slight pressure on the push so as to keep the ura even around the blade since pulling backwards naturally adds a downward force. As far as the scratches, you have to get some decent stones which leave that misty or kasumi look when finished. I use the gesshin 400- king 800- gesshin aoto on the blade road to achieve this then I finish the hagane and ura up to the rika 5k.

OK. Thanks. I will get back to you later.. after I've read some more background info

Very briefly, when you heat treat a steel the quench is intended to "lock" the structure. It's the carbon that allows that to happen, and as mild steel has a low C content it doesn't happen. With single sided laminates, there are stresses introduced where the high C steel is only supported on one side. This creates a tendency for the blade to warp/twist. The high C steel is slightly longer post HT than pre, and the mild doesn't really change.
The grinding will also affect the internal stresses causing the knife to bend/twist as these are released.

Keeping it straight involves more skill and steps in the process, which costs more money. So cheaper single bevel knives are more prone to being out of straight.

If you hold your knife by the handle edge up and look down it does it look straight? Try the same spine up?

If the knife has a bend or twist in it, when you sharpen it you can end up with gaps in the board contact like in your picture. Where Usuba's have an almost flat edge it's more of a problem.